Ex. Don't we love it when Harry Potter and Malfoy get into it? Or when Hansel and Gretel turn the tables on the witch?

Finally a best selling story needs conflict and a big problem that the main character needs to overcome.

Ex. If Harry lets Lord Voldemort come back without a fight, the fate of the magic world could be at risk.

Ex. If Violet, Klaus, and Sunny Baudelaire don't outsmart Count Olaf then their fortunes and their lives might be at risk.

Most inexperienced writers spend so much time thinking about the setting, the scenery and the color of their characters hair that they forget that the plot is what editors and their audience is looking for.

Hogwarts is a wonderful school. But who would care about it without Harry Potter and his friends.

The castle in Sleeping Beauty would just be another castle in the middle of nowhere without the princess and her prince.

And the three little pigs houses could have been made of snow, cotton or peanuts for all we would care without three clever little pigs and a wolf.

Kids love it when good triumphs over evil. Give them a story they can cheer over.

Also spend time really getting to know your characters. Create a history for each character, even if most of their histories will never see the inside of your book.

Your characters must seem real. Your audience must be able to relate to them and really care about what happens to them.

That in a nutshell is how you write a best selling children's book. The editing, minor scenic details and hand wringing anxiety can come after you finish the book.